1. Field of the Invention
The subject invention relates to a method of controlling a chassis dynamometer to simulate actual road conditions.
2. Description of the Prior Art
Chassis Dynamometers simulate real world driving situations in lab environments by simulating forces associated with driving a vehicle through various road conditions. Typically, a chassis dynamometer includes at least one roller in contact with tires of a vehicle. The roller transmits forces to or absorbs forces from the vehicle. An actuator attached to the roller drives or brakes the roller during a simulation.
The actuator includes a load cell that measures force exerted by the motor. A separate sensor usually measures rotational displacement, speed, and acceleration of the roller. The sensor and load cell provides feedback to a control system. The control system provides input to the actuator to attain a desired force between the vehicle and the roller, i.e., either driving or braking the roller. Tractive effort is a term of art describing the force between the roller and the vehicle. The required tractive effort for a specific simulation must be input into the control system algorithm to convert the tractive effort into the proper rotation of the actuator and roller.
The tractive effort has been traditionally determined by a road load equation. The road load equation determines the forces required to accurately simulate the vehicle on the road. Forces encountered by a vehicle on the road include inertial forces, breakaway friction, and drag forces caused by the vehicle passing through the air. The road load equation is common to the art and takes these forces into account in determining the overall forces a chassis dynamometer must exert on the vehicle to accurately simulate real world driving conditions. The measured road load forces are represented in the following equation:
Road load=A+(BV)+(CV2)
where A=a constant force coefficient, B=a variable force coefficient, C=windage or drag; and V=velocity of the vehicle. The road load represents the output required by the chassis dynamometer to accurately simulate actual driving conditions, i.e., the tractive effort. The control system converts the road load or tractive effort into instructions to the actuator to determine how quickly to rotate the rollers to attain the desired force between the roller and the vehicle. Due to the physical limitations of the actuator, roller, vehicle, etc., the desired force can never be instantaneously obtained. Hence, the need for feed back and feed forward or a combination of feed back and feed forward systems to eventually obtain the desired force or tractive effort.
A typical feed back control system utilizes feedback from the load cell and sensors to progressively step the actuator and roller to the desired tractive effort. The feedback control system includes a proportional, integral and derivative, or PID, controller. The PID controls the actuator based on the difference between the input PID reference and feedback. The output of the PID directs the actuator to move to a higher level of force. This process is repeated continually to move the chassis dynamometer incrementally to the desired target force or tractive effort. These typical feed back control systems, however, have a number of deficiencies. Some examples of the feed back control system""s deficiencies are slow response times and oyershoot of the desired target force.
A feed forward control system directly inputs to the actuator. Therefore, the feed forward system goes directly to the desired target force without incremental adjustment like the feed back system. The actuator, however, is still not as accurate as required for road load simulations. Therefore, a PID is added to eliminate the differences in actuator response. The PID utilizes the feed back from the load cell and the sensor. The advantage of a feed forward system is a quicker response time than a feed back control system. However, the disadvantage is typically an overshoot of the target force. The overshoot is due to the actuator operating on the feed forward portion of the signal at the same time the PID is building error and increasing an overall drive reference. To counter the overshoot an error switch is sometimes employed. The error switch will shut off the direct actuator reference at a point short of the target force and then allows only the feed back portion of the control system to send input to the actuator.
Another improvement to chassis dynamometer control systems is disclosed in the U.S. Pat. No. 5,465,612 to La Belle. The La Belle ""612 patent improves on the combined feed forward, feed back method by accounting for frictional and other parasitic losses in the drive and roller. Parasitic loss data is sensed at the roller and combined with conventional torque and speed data and then feed into the PID controller. Drive signal output from the PID controller combines with the feed forward signal to improve response time by accounting for losses inherent in the chassis dynamometer drive. However, this technique, still provides less than optimum results.
The accuracy of the control system is critical to properly simulate real world driving conditions. New specifications for these simulations require control systems with increased accuracy and faster response times than historically has been available. For these reasons it is desirable to design a control system that can attain desired target forces quicker and in predictable ways for all weights of vehicles and types of chassis dynamometers.
A method of controlling a chassis dynamometer to simulate actual road conditions experienced by a vehicle is disclosed. The chassis dynamometer includes at least one actuator coupled to a roller and a controller to control the actuator. The method includes rotating the roller to attain a predetermined target force between the roller and the vehicle. The method is characterized by establishing a mathematical model of the target force between the roller and the vehicle and rotating the roller in accordance with the mathematical model.
The use of models to predict chassis dynamometer behavior based on a given controller input increases accuracy and decreases response time. Predictive models of chassis dynamometer behavior provide faster and more accurate control. Predictive models also enable a control system to control the chassis dynamometer to obtain a desired response in a desired way to achieve expected results. Additionally, predictive models allow emulation of physically different chassis dynamometers.